Solution and solid-state conformations of 1,5-pyridine and 1,5-phenanthroline-bridged p-tert-butylcalix[8]arene derivatives

An extension of the method for the regioselective introduction of nitrogen-containing heterocycles at the 1,5-phenolic positions of p-tert-butylcalix[8]arene previously described for 2,6-bis(chloromethyl)pyridine allowed the use of 2,9-bis(bromomethyl)-1,10-phenanthroline as a selective 1,5-dialkylating agent. The 1,5-dimethylpyridine derivative was previously characterized as a relatively rigid dicesium complex [1(4H)2−·2Cs+], its metal-free counterpart 1(6H) is herein described as a fluxional species in solution, as evidenced by VT–NMR and theoretical studies. The 1,5-[2,9-bis(methylene)-1,10-phenanthroline]-p-tert-butylcalix[8]arene analogue 2(6H) was characterized spectroscopically in solution as a relatively rigid cone conformer at 390 K. The identity of both metal-free 1(6H) and the monocesium complex [2(5H)−·Cs+] was established in the solid state by X-ray crystallography, confirming the 1,5-substitution of the latter. The conformational behavior of 1(6H) and 2(6H) in solution is described in terms of the enthalpic and entropic contributions by semiempirical methods.